The present invention relates to nuclear reactors whose core is cooled by a liquid metal generally constituted by sodium contained in a vessel having a vertical axis and open at its upper end, the invention relating more specifically to fast neutron reactors.
In the classic design of such reactors, the core and the liquid sodium volume which cools the same are located in a vessel having a vertical axis and which is open at its upper end, being suspended beneath a thick protective slab which seals the vessel, whereby passages are provided within the same to give access to the core or to other components necessary for operation such as exchangers and pumps carried by the slab and distributed about the core. These components ensure on the one hand the transfer of the calories absorbed by the liquid metal after traversing the core and on the other the continuous circulation thereof. Advantageously the core is itself placed in a second vessel, called the inner vessel, which separates the liquid metal volume in the first vessel, called the main vessel, into two regions respectively called the hot collector and the cold collector. The hot collector collects the liquid metal when it leaves the core and after said liquid metal has been cooled by passing through exchangers it is collected in the cold collector and then taken up by circulating pumps which return it under an appropriate pressure to the reactor core in accordance with a continuous cycle.
According to another known arrangement the core formed by the juxtapositioning of fuel assemblies rests on a supporting member constituted by two plates or supporting grids having cavities in which vertically engage the feet of said assemblies. The pumps deliver the cold liquid metal to the supporting member and the junction between the feet of the assemblies and said member a leakage flow is provided which is small compared with the main flow passing through the assemblies for ensuring the cooling thereof. This tapped flow is recovered beneath the supporting member of the core and passes to an annular space, defined between the inner wall of the main vessel and a baffle which is parallel thereto where it circulates and cools the side wall of the vessel before being returned to the cold collector in which it mixes with the remainder of the liquid metal volume. The leakage flow can be returned to the cold collector either by reversal above the upper edge of the baffle with a return between said baffle and a parallel counter-baffle, or by flowing freely over the same edge into the collector.
In both these solutions the cooling liquid sodium in the upper open part of the vessel is in contact with a neutral covering gas above the liquid metal level in the vessel beneath the slab which seals the reactor. Thus difficulties can occur during the variation in the level of the free sodium in the vessel. Thus, if it is arranged so that the removed liquid sodium reversal level is permanently maintained below that existing in the vessel, there is no risk of entrainment of said covering gas, whereas with low level operation of the reactor when the removed liquid sodium level is maximum, stagnant uncooled layers are formed in the upper part of the vessel between the latter and the baffle. However, when it is arranged that the overflow level of the liquid sodium removed is permanently above the maximum level in the vessel, the cooling of the latter is ensured under all operating conditions but the overflow obtained may then cause a disadvantageous entrainment of the covering gas.